This invention relates to wire chambers such as single or multi-wire proportional counters, drift chambers, Geiger tubes and the like. These devices are commonly used to detect the passage of a photon or highly energetic subatomic particle through a closed chamber by detecting ionization of a gas within the chamber, caused by absorption of a photon by a gas molecule or a collision between a gas molecule and a subatomic particle.
In general, such wire chambers use at least one anode wire and cathode wires stretched through a chamber filled with a predetermined gas, the valence electrons of which are liberated by absorption of a photon or a collision between a gas atom and a subatomic particle. Electrons liberated from the gas atom or molecule cause the liberation of other free electrons by collisions in a chain-reaction such that an avalanche of free electrons is produced by the initial collision or photon absorption to produce a detectable current surge in one of the electrode wires. All such wire chambers therefore require at least one gas that provides for electron production resulting in an avalanche of such electrons by the absorption of a photon or a collision between an atom of the gas and a subatomic particle of interest. Most wire chambers use at least one additional gas constituent to absorb (quench) ultraviolet photons for prevention of secondary avalanche formation and to stabilize the speed of the avalanche of electrons to improve the timing accuracy of the instrument by providing a more constant velocity of the avalanching electron drift speeds.
It has been found and disclosed that a mixture of argon and ethane gas provides an acceptable electron production characteristic and an acceptable electron drift velocity stabilizing characteristic. See Nucl. Instr. and Methods 156 (1978) 163-168 by M. Atac and J. Urish. and see Nucl. Instr. and Methods in Phys. Res. A 249 (1986) 265-276 by M. Atac et al.
However, a significant problem with wire chamber gas mixtures, has been the additional requirement that ultra-violet photons that penetrate the chamber from a variety of sources across a wide range of wavelengths, be absorbed by the gas, without producing additional ionization in the chamber that would indicate false collisions or events occurring within the chamber. Specifically, energetic photons that strike the cathode wire in a wire chamber will produce free electrons which will result in the production of another avalanche of electrons which will be detected at the anode as another event. These UV photons which strike the cathode wire, if not absorbed by other gases in the chamber, will produce repetitive false counts at the anode wire.
A characteristic of the gas mixture used in wire chambers must therefore be the ability to absorb ultraviolet wavelength photons preventing the false counts that would result from the absorption by the cathode of these UV wavelength photons. It has been found that certain alcohol vapors provide long wavelength UV absorption, which when used in combination with ethane, provides shorter wavelength absorption, the full range of UV wavelengths can be effectively absorbed before the photons impinge the cathode wires used in the wire chambers. See I.E.E.E. Transactions on Nuclear Science, Vol. NS-31, No. 1 (1984) 99-102 by M. Atac.
A substantial problem however in using gas mixtures such as argon and ethane with ethanol or methanol has been that the dissociation products of these alcohols, which are produced by the absorption of a long wavelength ultra-violet photon, are corrosive to the metals used in the wire electrodes and shorten the electrodes useful life by oxidizing the outer layer of the wire surface. Certain metals such as gold, platinum and stainless steel are more resistive to the corrosive dissociation products of ethanol and methanol but are substantially more expensive than other metals that would be useable in a wire chamber were it not for the rapid aging caused by the use of ethanol or methanol with argon and ethane. In complex wire chambers, such as radial wire drift chambers, replacement of wires is prohibitively expensive and time consuming.
It is desirable therefore to be able to operate a wire chamber with a constituent gas mixture that provides a stable electron drift velocity, with the ability to absorb all UV wavelengths without producing dissociation products which cause premature aging of electrode wires.
It is therefore an object of the present invention to provide a wire chamber improved with a gas mixture which prevents premature aging of electrode wires. It is another object of the present invention to provide a wire chamber improved with a gas mixture that provides stable electron drift velocities and good quenching of UV photons across a wide range of UV wavelengths.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.